TH400 Transmission Rebuild - Trans-Former

With all the emphasis on g-machines, 17-inch wheels and making vintage cars handle like new ones, sometimes it's easy to lose sight of the street-strip cars that still make up a huge portion-perhaps the majority-of the hobby. They don't have fuel-injected LS engines under the hood or electronically-controlled transmissions. Heck, most don't even have overdrive. Just big tires out big, smaller ones up front and lots of horsepower under the hood.

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In future issues, you'll be reading about a '72 Nova SS we're building that's very similar to the vehicle described in the preceding paragraph. It's mini-tubbed, set up for a big-block and we procured it minus the engine and trans. Since it had been home to a Rat for many years, we figured it should get another. To that end, we're making use of the Dart 509 you read about a few issues back (Real Fine 509, November and December 2009 issue). To review, Dart took its Big M block and punched the cylinder bores to 4.500-inches each. Then it took a standard Eagle 4.00-inch stroke crankshaft and came up with 509ci. The top-end consists of a Comp Cams hydraulic roller bumpstick, Dart Pro 1 310cc heads, and Dart single-plane intake manifold. All told, this baby cranked out a stout 668 hp and 623 lb-ft-no doubt, this puppy will be a nasty little street machine once finished.

The Dart 509 power curve dictates the need for a strong unit in the trans tunnel. For that, we skipped over the popular TH350 in favor of the stronger TH400. The GM factory officially refers to a 400 trans as a Turbo-Hydramatic (TH) and it first appeared in the GM lineup back in '64. The 400 moniker's origins are unknown. The transmission had a long and successful life in a variety of GM vehicles until it was phased out sometime in the '80s, giving way to the four-speed overdrive transmissions. In the world of muscle cars, this transmission was found behind many a muscle car, including big-block Chevelles and Corvettes. A lot of racers still use the Powerglide two-speed, but we opted for the TH400 over a 'Glide because of its three-speed classification-making it fun on the street. The three forward gears consist of a 2.48 first gear, 1.48 second gear, and a final 1.00 ratio for third.

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We didn't just dig a TH400 out of the junkyard and throw it in this X-body; our plan pointed us to Gaithersburg, Maryland, which serves as home to Performance Automatic (PA). The folks at PA took our junkyard TH400, cleaned it up, and turned it into its Street Smart Stage II transmission. The Street Smart Stage II is rated to 650 rear wheel hp, which is plenty for our big-block powerplant, even if we decide to hit it with a shot down the road. Our engine cranked out 668 hp at the crank, which differs from rear-wheel hp by roughly 15-20 percent loss through the drivetrain. PA also offers a few other versions of the TH400 with varying hp ratings and different options to fit most budgets.

Our junkyard slush box was not in stellar shape and PA disassembled and inspected all of the components before rebuilding it. Perhaps the only bump in the road that PA's Gerald Nusser discovered was a broken linkage. "This is rare but it is something we fix by just welding it. This case was fixable, but in other scenarios, we will discard the case rather than attempt to fix it," he stated. PA keeps a vast inventory of TH400 transmissions (as well as other trans styles) in the event a customer has an irreparable case or simply doesn't have a core to trade in.

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The Stage II trans features better shift calibrations, heavy-duty Alto Red clutches and bands, and upgraded sprags. In addition to those mods, PA also added a new transmission pan and drain plug. The biggest highlight of the Stage II packages is the forward and direct drum each carrying five Alto Red high-performance clutches. The stock drums contain only four clutches.

Our junkyard slush box is now a solid transmission that is ready, willing, and able to hold up behind the Dart 509ci. Tune in to future issues as we continue to grind away on this unsuspecting Nova.